This is a revision of the test_intermixed.py that I submitted earlier.  Now `parse_intermixed_args` acts like `parse_args', and calls `parse_known_intermixed_args`.  Again it is form that can exercise the idea without modifying `argparse.py`.

If the parser has incompatible features (REMAINDER, PARSER, or certain exclusive groups), it raises an error.  However to facilitate testing I included a `_fallback` backdoor.  If not default None it will be called instead of raising the error.

While making documentation changes, I got to wondering whether 'interspersed' would be a better term than 'intermixed'.  optparse has an 'interspersed' option and api.  However the getopt documentation does use 'intermixed'.